FIELD OF THE INVENTION
This invention relates to telephone devices for subscriber systems, in particular telephone subscriber devices adapted to be monolithically integrated.
BACKGROUND OF THE INVENTION
In telephone circuits of the one-chip type which serve multiple functions normally performed by discrete integrated circuits such as modern speech, ring, and tone circuits--the system common point of reference, both physically and electrically, is an individual one coinciding with the substrate of semiconductor material whereto the whole one-chip circuit is integrated.
In a telephone subscriber system, the telephone subscriber speech circuit and ring circuit are both coupled to, and energized through, the telephone line. During the ring phase, however, any coupling of the two circuits together must be prevented, as by disconnecting the speech circuit through switches, whereas during the speech phase, the ring circuit must be disabled. Current telephone company standards, moreover, call for the ring circuit to be DC uncoupled from the telephone line for any line polarity. This requires, as is well recognized by the skilled ones in the art, either the provision of a dual RC network, or the inclusion of a traditional electric switch which can be readily integrated. Understandably, the last-mentioned solution is the one which would normally be utilized wherever the telephone circuits involved are of the one-chip type.
In a telephone subscriber system, the speech phase is initiated by connecting the telephone line (DC connection) to the telephone subscriber speech circuit as the handset is picked up or, in more sophisticated designs, on a pulse being issued from a keyboard. But, if one wishes to connect the line to the telephone subscriber speech circuit without picking up the handset and without acting physically on the telephone apparatus as by keying in--operation from an external electric control being preferred instead--problems may arise from the line polarity being undefined and the line's own reference potential being other than the reference potential of the one-chip circuit, i.e., its substrate. In fact, an input terminal of the device--namely that connected to the terminal of the polarity bridge, which would usually be a diode bridge enabling the speech circuit to be coupled to the line--might take a negative potential with respect to the system reference, i.e., the device substrate.
To obviate the difficulties brought about by the different electrical references, it has been common practice to employ in most cases either electromechanical devices or opto-electronic components, which are effective but cost-intensive.
SUMMARY OF THE INVENTION
The underlying technical problem of this invention is to provide a telephone subscriber system with a monolithically integrated line switch, for driving by external electrical signals, which can enable the speech circuit even under a condition of differing telephone line and subscriber system reference potentials.
This problem is solved by a telephone subscriber system having a circuit interface means connected to a common reference potential driving a pair of switches, where the circuit interface means includes a unidirectional current flow circuit element to control drive currents produced in the circuit interface means.
The features and advantages of a telephone subscriber system according to the invention can be appreciated from the following description of an embodiment thereof, to be taken by way of example and not of limitation in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram, partly in block form, of a telephone subscriber system according to the invention.
FIG. 2 is a circuit diagram of a possible embodiment of a circuit interface means portion of the telephone subscriber system of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
The circuit illustrated by the diagram in FIG. 1 comprises first diode bridge PP, and second diode bridge PR adapted to respectively couple a speech circuit SPC and a ring circuit RIN to a telephone subscriber line represented by a block SL. Each bridge circuit has a respective first terminal 30, 31 and second terminal 32, 33 for coupling to the line. The second bridge circuit PR is, however, DC uncoupled from the line by means of a resistor R and a capacitor C. The second bridge circuit PR functions as a rectifying bridge for the ring signals, whereby the ring circuit is activated to operate the bell of the subscriber set. On the other hand, the bridge circuit PP functions to maintain, on its two output terminals ("+" and "-") 34, 36 thereof for coupling to the speech circuit, a predetermined polarity irrespective of the line polarity.
Connected across the two output terminals 34, 36 is a first circuit switching means, SW1, adapted to be actuated by the user picking up the handset or causing a pulse to be issued from a keyboard. The first circuit switching means SW1 drives two more circuit switching means, SW2 and SW3, whereby the speech circuit SPC is coupled, over a terminal pair, to the "+" and "-" terminals 34, 36 of the bridge circuit PP. The above three changeover circuit means SW1, SW2, SW3 act as switches effective to disconnect the speech circuit SPC from the subscriber line SL during the ring phase.
A fourth circuit switching means, SW4, driven by SW1 acts as a switch effective to disable the ring circuit RIN by breaking the connection between the ring circuit RIN and the second bridge circuit PR upon the first circuit switching means SW1 activating the second and third circuit switching means SW2 and SW3 and accordingly enabling the speech circuit. The circuit switching means SW1, SW2, SW3 and SW4 are implemented as MOS transistors in the preferred embodiment. Alternatively, other implementations, such as bipolar junction transistors may be used where appropriate.
Also shown in FIG. 1 is an actuator circuit represented by a block ACT. The actuator circuit ACT comprises the required circuit parts to actually control the circuit switching means SW2, and is connected between the "+" terminal 36 of the first bridge circuit PP and a circuit communication node G establishing a link between the speech circuit SPC, the ring circuit RIN and the third circuit switching means SW3. The actuator circuit ATC is a known circuit that is publicly available to those of ordinary skill in the art. The actuator circuit ATC provides the signals for driving the switching means SW2, and SW4 to allow the use of the telephone device during the speech or ring phases. The input signal of the actuator ACT is supplied by the switching means SW1 when picking up the telephone handset or touching a key on the keyboard. The driving command on the switching means SW2, SW4 is running only if the third switching means SW3 has been previously activated. Any suitable circuit that is presently well known is acceptable; of the many acceptable circuits, one known circuit is presented in FIG. 12 of the published data manual for commercially available products TDE 1747-TDF 1747 from SGS Microelectronics.
For telephone devices of the one-chip type, said circuit communication node G is a representation circuit-wise of the semiconductor material substrate, whereto the speech circuit SPC and ring circuit RIN are integrated monolithically, constituting the reference potential for the telephone subscriber system.
Unlike the third circuit switching means SW3, which is also connected to the circuit communication node G, the circuit switching means SW2 is connected to no reference potential and, therefore, it is convenient to provide, with current circuit techniques, an actuator circuit ACT having as its reference potential that of the substrate of the single chip represented as node G.
In accordance with this invention, the circuit in FIG. 1 comprises a circuit interface means CI having an input terminal E for the application of external electrical signals in relation to the circuit node G, and output terminals for driving the second circuit switching means SW2 through the actuator circuit ACT, as well as driving the third circuit switching means SW3 directly.
An essential feature of the invention is that such circuit interface means CI include a circuit unidirectional current flow element, shown as a diode D1, connected, in parallel with the third circuit switching means SW3, between the "-" terminal 34 of the bridge circuit PP and the circuit node G.
In a telephone subscriber system according to the invention, an external electrical command referenced to the substrate potential and the operation of the high-voltage electronic switch SW3 referenced to the negative potential of the polarity bridge circuit PP are properly interfaced in quite a simple manner which may be realized in a fully integrated form. The operation of switch SW3 is enabled by the provision of diode D1 between the bridge circuit PP and the substrate. The drive currents produced in the interface circuit CI by the external electrical command flows to the substrate, via the diode DI, under any polarity conditions.
The electronic switch SW3 may be implemented, in a known manner to those skilled in the art, using a single NPN transistor in a Darlington configuration, or alternatively, a transistor of the MOS type, depending on the integration technology employed. Shown in FIG. 2 is a preferred embodiment of the electronic switch SW3 and the interface circuit CI for driving by external electrical commands related to the system common node G. The switch SW3 is implemented by an N-channel DMOS component, and the interface circuit CI is also made up of components of the MOS type. Of course, bipolar components would be employed in embodiments incorporating a bipolar type of electronic switch.
FIG. 2 shows a resistor R1 and a Zener diode Z1 connected across terminals A and B of the switch SW3; the resistor R1 ensures that the electronic switch SW3 is normally open when no commands (by picking up the handset or keying in) are applied to the terminal B, whilst the Zener diode Z1 between the gate and the source of the N-channel DMOS transistor functions as a gate voltage protection.
The interface circuit CI comprises a current generator consisting of a current mirror circuit 100 whose output leg D drives both the actuator circuit ACT and the electronic switch SW3 through terminal B. Provided between the input leg of the current mirror circuit 100 and the system communication node G is a MOS transistor 102 whose gate terminal 104 is connected, via a diode 106, to the current mirror circuit 100 as well as to the input terminal E to which external electrical commands referenced to the common node G, i.e., the substrate, can be applied. In this embodiment, a diffusion of the P type is "seen" at the output D of the interface circuit CI. This is important because whenever line polarity conditions are encountered, in the absence of any command, whereby the potential at point A is lower than that at point G--which is the system common point (substrate)--the junction is reverse biased with respect to the substrate and, therefore, the telephone set isolation resistance kept high. The output current flows into R1 from point D, and loops to the substrate through the diode D1. The voltage drop across R1 drives the transistor of switch SW3 into conduction, and allows current to flow between the "+" and "-" terminals 34, 36 of the bridge circuit PP. The voltage drop across resistor R1 also causes, the switch SW2 to conduct, through the actuator circuit, thereby enabling the speech circuit SPC and the single chip as a whole to be energized and operated.
The operability of the telephone circuit connected to the line can thus be enabled by means of electrical commands which have for their reference the system common point G and may be supplied, for example, from an automatic answering set, as connected or incorporated to the telephone subscriber system. It should be noted that the diode D1 is intrinsic to the monolithically integrated N-channel DMOS component, so that this embodiment of the invention, in addition to being uniquely reliable, is far less expensive than prior art solutions.